The magnetic parts of the stator and the rotor of an electrical machine usually comprise magnetically conductive sheets stacked on top of each other to form a sheet pack. The sheet-type structure ensures a good flow of magnetic flux and prevents the formation of eddy-current losses, for example. A stator and rotor pack manufactured from sheets is preferable in terms of manufacturing technique, because any holes parallel to the motor shaft that have to be arranged in the pack, such as those made for the shaft, winding grooves, cooling tubes and longitudinal fastening bolts, are easy to manufacture by die cutting corresponding holes or cavities in the sheets. Similarly, it is easy to make the circumferential surface of the stator and rotor circular or, if desired, slightly deviating from circular.
In permanent-magnet electrical machines, the permanent magnets are generally fitted to the rotor either by embedding them into the rotor sheet pack or fastening them onto the circumferential surface of the rotor. When the permanent magnet pieces are embedded into the rotor sheet pack, corresponding grooves, as known from prior art have to be made in the sheet pack. Today, efficient permanent magnets are manufactured from neodymium iron boron (NdFeB), which has excellent magnetic properties. Even though it is not as fragile as samarium cobalt that was generally used previously, the handling of permanent magnet pieces made from it and the fitting of them into the designated grooves requires quite a great deal of precision and time. The high magnetic force of new permanent magnets also males their handling more difficult when they are fitted into place in a magnetised state.
The magnetic circuit of the rotor must be dimensioned so that the magnetic flux is routed through an air gap to the air gap of the electrical machine and further to the stator in a way that is as optimal as possible. Simultaneously, the rotor has to be strong enough to bear the loads imposed on it. Because the rotor is a rotating part, its construction must be as light as possible. The sheet pack, comprising magnetically conductive iron sheets, permanent magnet material and any other body structures and the shaft of the rotor must form a sufficiently strong and uniform unit within the completed machine. The machine must also be easy and inexpensive to implement in terms of manufacturing technique.
An electrical machine including a stator and having permanent magnets embedded into the rotor is known from the publication WO 00/62399. However, the solution according to the publication has significant shortcomings with regard to the properties described above.
The purpose of the invention that is the subject of this application is to create a new permanent-magnet motor that is inexpensive and easy to manufacture, with magnetic properties that are competitive and compliant with current requirements. In order to achieve this, the invention is characterised by the features specified in the characteristics section of Claim 1. With regard to other preferred embodiments of the invention, reference is made to the dependent claims.
The solution according to the invention creates a permanent-magnet electrical machine with a rotor that is inexpensive to manufacture. The parts of the rotor can be easily handled during manufacture and conveniently combined with each other. The magnetic sheet pack or partial sheet pack comprising the magnetic circuit of the rotor is an independent compact unit before the permanent magnets are fitted to it. The permanent magnets can be installed and attached in place without the need for special tools. The first sheet pack, which is close to the central axis of the rotor's revolution, is supported by the rotor shaft and extends close to the outer circumference of the rotor between the poles. This creates cavities essentially equal in width to the rotor poles on the side of the rotor's circumference. The poles are formed from other sheet packs fitted in the cavities of the first sheet pack and essentially extend to the circumference of the rotor. This creates the outer surface of the rotor pole. Space remains between the first and second sheet pack, extending to the outer circumference of the rotor with essentially equal width. The permanent magnets intended to magnetise the motor are fitted into this space. Installation is preferably carried out from the outer circumference by pushing the magnets into place.
It is simple to secure the fastening of the permanent magnets in the radial direction of the rotor, and no special tools or parts are required.
According to a preferred embodiment of the invention, the rotor comprises several axial modules. This makes it possible to manufacture even a large motor without any complex arrangements. Modular dimensioning facilitates design and brings benefits in terms of manufacturing technique. Production facilities can be used in a functional way and the permanent magnets can be magnetised separately for each module.
According to another preferred embodiment of the invention, the outer sheet pack facing the rotor's air gap and the inner sheet pack attached to the rotor shaft are made from different types of sheets. The sheets can be chosen on the basis of technical and economical factors related to the different properties particularly suitable for each pack.
According to yet another embodiment, uniform sheets with no openings for permanent magnets are arranged between the partial modules. This makes the mechanical properties of the rotor even better.